The difference is that the M4 is 3-year old tech. Spec-wise they may seem similar however real-world there is little comparison. The M500 is hands down faster on paper and in practice. They have some enterprise features baked right in like capacitors to enable it to finish writes in the event of a power failure. Also they have a hardware-based AES-256 encryption engine to protect your data from prying eyes. If u need a new SSD, the M500 is waaaaaaaaaay better.
Please read the following Question Regarding your laptop Thinkpad T400
and the Crucial M500 thickness is 9.5mm
so I think it can replace your Main Disk without any problem.
for the external you will need additional Kit.
If you want to transfer the OS and all of your apps and data from your old hard drive to your new one, you will need software like Apricorn's EZGig and a usb to eSATA cableto hook up the new hard drive to a usb slot on your old computer. Once you transfer all of the info from the old to new hard drive, you turn off the old computer, disconnect the battery if possible, and then open up the computer to switch out the hard drives. If you aren't interested in preserving your old data and have OS and all of the drivers for your computer, you can install the new hard drive and then set it up from scratch by installing the OS and then all drivers and applications you desire. Make sure you have all of the installation discs or downloads for the applications you want to reinstall if you don't do a drive copy.
Physically you will need a 2.5 to 3.5 mounting adapter and a SATA connection.
I personally recommend installing the OS from scratch and then copying the data over from the old drive. If that is not practical then use Ghost, Acrounis or other imaging software. If you do image then there are a number of important setting adjustments that need to be made that are beyond the scope of this reply that can be found at http://thessdreview.com/ under Guides.